When driving a current output (e.g., 4-20 mA current), a minimum compliance voltage, and therefore supply, is needed. In a fixed power controller, the compliance voltage is typically set to a fixed voltage based on a predetermined maximum load and the maximum current that will be driven into that load. As the current output converter does not know the load, the power supply compliance (and therefore the power supply) needs to be large enough to cover the whole load range. If the actual load is less than the maximum, then the excess power would get dissipated on the power controller and in the housing of the driving device. Typically, the compliance voltage is set to the minimum compliance voltage required plus a small headroom voltage, e.g., 2V.
When operating in a current output mode, a converter with dynamic power control capability such as the AD5755 family of devices sold by Analog Devices, Inc. of Norwood, Mass. can be configured to sense the needed output compliance voltage and to dynamically change the power supply voltage to meet compliance requirements, e.g., using an on-board DC-DC boost converter to modulate the power supply as needed. For example, using the formula VOUTmin=IOUTact×RLOADmax (where VOUTmin is the minimum required output voltage, IOUTact is the actual current output, and RLOADmax is the maximum remote load), if RLOADmax=1 Kohm and IOUTact=10 mA, then VOUTmin=10V; but if RLOADmax=1 Kohm and IOUTact=20 mA, then VOUTmin=20V. Typically, the output voltage of the power supply is set to VOUTmin plus a small headroom voltage, e.g., 2V. Such dynamic power control is discussed, for example, in The Analog Devices AD5755-1 Data Sheet Rev. B (2011) available at www.analog.com/static/imported-files/data_sheets/AD5755-1.pdf, which is hereby incorporated herein by reference in its entirety. Among other things, such dynamic power control using a dc-to-dc boost converter circuit reduces power consumption compared to standard designs when using the part in current output mode.
One drawback of such dynamic power control is that, as the power supply is modulated directly from the current draw, if large output swings are needed, the settling time of the current output can become very large, especially when driving into high load values. Thus, while such dynamic power controllers tend to improve power dissipation, they often do so at the expense of the settling time.